Currently, demands for downsizing of piezoelectric devices, as well as semiconductor devices, are, increasing with the development of the microprocessing and micromachining. For example, piezoelectric devices constituted by a piezoelectric body and electrodes are used as, for example, actuators installed in inkjet recording heads. In the piezoelectric devices, the piezoelectric body expands and contracts in correspondence with increase and decrease in the strength of an electric field applied from the electrodes to the piezoelectric body. In order to improve the image quality, it is necessary to increase the installation density of piezoelectric devices constituting each inkjet recording head. In the case where high installation density of the piezoelectric devices is required, it is preferable to minimize the thickness of the piezoelectric body used in each piezoelectric device from the viewpoint of the precision of the processing or machining. Therefore, there are demands for thin piezoelectric films exhibiting satisfactory piezoelectric performance.
For example, perovskite oxides such as PZT (lead titanate zirconate) are known as materials suitable for the piezoelectric body. Such materials are ferroelectric materials which exhibit spontaneous polarization even when no electric field is applied. The piezoelectric materials are reported to exhibit high piezoelectric performance at and near the morphotropic phase boundary (MPB).
Currently, the public interest in the environmental load is increasing, and demands for lead-free piezoelectric films are increasing. Various types of materials including the following types (1) to (3) are currently being studied as lead-free perovskite oxides exhibiting superior piezoelectric performance.
(1) The +1/+5 type perovskite oxides in which one or more monovalent metal elements (including K, Na, and Li) reside in the A sites, and one or more pentavalent metal elements (including Nb, Ta, and Sb) reside in the B sites.
(2) The +2/+4 type perovskite oxides in which one or more divalent metal elements (including Ba and Sr) reside in the A sites, and one or more quadrivalent metal elements (including Zr and Ti) reside in the B sites. The +2/+4 type perovskite oxides also include a type in which the trivalent metal element Bi and the one or two monovalent metal elements Na and/or K reside in the A sites so that the average valence of the A-site elements becomes two.
(3) The +3/+3 type perovskite oxides in which one or more trivalent metal elements reside in both of the A sites and the B sites. For example, a piezoelectric device having a piezoelectric film of BiFeO3 is disclosed in Japanese Unexamined Patent Publication No. 2005-039166 (which is hereinafter referred to as JP2005-039166A, and corresponds to the U.S. Pat. No. 7,216,962).
However, the conventional piezoelectric films of lead-free perovskite oxides do not have a piezoelectric constant d31 exceeding 50 pm/V. For example, although BaTiO3 is known as a lead-free perovskite oxide exhibiting high piezoelectric performance, the piezoelectric constant d33 of the BaTiO3 film is approximately 120 pm/V, which corresponds to the piezoelectric constant d31 of approximately 50 pm/V. In addition, the BiFeO3 film is also known as a piezoelectric film. However, since iron (Fe) as the B-site element is a transition metal element, the valence of Fe is likely to vary, so that the leakage current can increase.